The present invention relates to a method for operating a steam stripped fractionation column to prevent water condensation on the fractionation trays located in the top of the column thereby obviating operational upsets due to flooding or other undesirable effects such as corrosion.
Many different applications in the hydrocarbon refining and petrochemical industries employ the use of steam strippers to remove lower boiling compounds from liquid streams containing various boiling range compounds. The introduction of steam into a steam stripped fractionation column is beneficial for the separation of different boiling compounds, however, if too much steam is added for the amount of heat available in the column, steam will condense on the stripper trays where water builds up and eventually floods the stripper causing major operational upsets. The presence of liquid Water also leads to increased corrosion of the trays and walls of the stripper column. The present invention helps to prevent undesirable condensation of steam by providing the operator with an alarm that warns of conditions that approach the dew point so that the appropriate adjustments can be made before the stripper column is upset.
An article by E. W. Ellerbe, titled xe2x80x9cSteam Distillation Basicsxe2x80x9d and published in Chemical Engineering on Mar. 4, 1974, discloses that steam distillation offers a solution to some very difficult chemical-processing problems.
U.S. Pat. No. 6,106,785 (Havlena et al.) discloses a batch polymerization process controller using inferential sensing to determine the integral reaction heat which in turn is used to indicate the degree of polymerization of the reaction mixture batch.
The invention uses computers of well-known types to process the relevant information from a steam stripped fractionation column to determine the approach to a possible flooding problem. The column flooding begins when the dew point of water is reached in the top of the column and steam condenses to form water on the upper trays in the column.
One of the parameters or inputs is a measure of the total moles of hydrocarbon passing through the top of the column which includes the net overhead vapor and liquid and the reflux flow. The molecular weight of the hydrocarbon must be determined. The second parameter is the total moles of water in the overhead stream as measured by the flow of sour water from the overhead receiver. The mole fraction of water is the moles of water divided by the total moles of hydrocarbon in the overhead streams. The temperature and pressure of the column overhead must be known. The partial pressure of water is calculated as the product of the mole fraction of water and the column overhead pressure.
The computer has storage banks or memory which contains the thermodynamic properties of water and are commonly referred to as steam tables in paper form. With the hereinabove-described parameters and the thermodynamic properties of water, the dew point temperature of water in the column overhead can be determined. A comparison between the calculated dew point temperature of the water vapor in the column overhead and the temperature in the column overhead indicates whether or not the initiation of flooding is occurring.
One embodiment of the present invention is a method for operating a steam stripped fractionation column with the aid of a digital computer comprising: (a) measuring and providing the total moles of hydrocarbon passing overhead in the steam stripped fractionation column to the computer; (b) measuring and providing the total moles of water as steam passing overhead in the steam stripped fractionation column to the computer; (c) constantly determining the mole fraction of water as steam passing overhead in the steam stripped fractionation column; (d) measuring and providing the overhead pressure of the steam stripped fractionation column to the computer; (e) constantly determining the partial pressure of water by calculating the product of the mole fraction of water and the steam stripped fractionation column overhead pressure; (f) providing the computer with a database for the thermodynamic properties of water; (g) constantly determining the dew point temperature of the steam passing overhead in the steam stripped fractionation column; (h) measuring and providing the top temperature of the steam stripped fractionation column; and (i) rendering an alarm when the calculated difference between the dew point temperature determined in step (g) and the top temperature of the steam stripped fractionation column is less than a predetermined value.